The FGF family of proteins, whose prototypic members include acidic FGF (FGF-1) and basic FGF (FGF-2), bind to four related receptor tyrosine kinases. These FGF receptors are expressed on most types of cells in tissue culture. Dimerization of FGF receptor monomers upon ligand binding has been reported to be a requisite for activation of the kinase domains, leading to receptor trans phosphorylation. FGF receptor-1 (FGFR-1), which shows the broadest expression pattern of the four FGF receptors, contains at least seven tyrosine phosphorylation sites. A number of signal transduction molecules are affected by binding with different affinities to these phosphorylation sites.
Expression of FGFs and their receptors in brains of perinatal and adult mice has been examined. Messenger RNA all FGF genes, with the exception of FGF4, is detected in these tissues. FGF-3, FGF-6, FGF-7 and FGF-8 genes demonstrate higher expression in the late embryonic stages than in postnatal stages, suggesting that these members are involved in the late stages of brain development. In contrast, expression of FGF-1 and FGF-5 increased after birth. In particular, FGF-6 expression in perinatal mice has been reported to be restricted to the central nervous system and skeletal muscles, with intense signals in the developing cerebrum in embryos but in cerebellum in 5-day-old neonates. FGF-receptor (FGFR)4, a cognate receptor for FGF-6, demonstrate similar spatiotemporal expression, suggesting that FGF-6 and FGFR4 plays significant roles in the maturation of nervous system as a ligand-receptor system. According to Ozawa et al., these results strongly suggest that the various FGFs and their receptors are involved in the regulation of a variety of developmental processes of brain, such as proliferation and migration of neuronal progenitor cells, neuronal and glial differentiation, neurite extensions, and synapse formation.
Other members of the FGF polypeptide family include the FGF receptor tyrosine kinase (FGFRTK) family and the FGF receptor heparan sulfate proteoglycan (FGFRHS) family. These members interact to regulate active and specific FGFR signal transduction complexes. These regulatory activities are diversified throughout a broad range of organs and tissues, and in both normal and tumor tissues, in mammals. Regulated alternative messenger RNA (mRNA) splicing and combination of variant subdomains give rise to diversity of FGFRTK monomers. Divalent cations cooperate with the FGFRHS to conformationally restrict FGFRTK trans-phosphorylation, which causes depression of kinase activity and facilitates appropriate activation of the FGFR complex by FGF. For example, it is known that different point mutations in the FGFRTK commonly cause craniofacial and skeletal abnormalities of graded severity by graded increases in FGF-independent activity of total FGFR complexes. Other processes in which FGF family exerts important effects are liver growth and function and prostate tumor progression.
Glia-activating factor (GAF), another FGF family member, is a heparin-binding growth factor that was purified from the culture supernatant of a human glioma cell line. See, Miyamoto et al., 1993, Mol Cell Biol 13(7): 4251-4259. GAF shows a spectrum of activity slightly different from those of other known growth factors, and is designated as FGF-9. The human FGF-9 cDNA encodes a polypeptide of 208 amino acids. Sequence similarity to other members of the FGF family was estimated to be around 30%. Two cysteine residues and other consensus sequences found in other family members were also well conserved in the FGF-9 sequence. FGF-9 was found to have no typical signal sequence in its N terminus like those in acidic FGF and basic FGF. Acidic FGF and basic FGF are known not to be secreted from cells in a conventional manner. However, FGF-9 was found to be secreted efficiently from cDNA-transfected COS cells despite its lack of a typical signal sequence. It could be detected exclusively in the culture medium of cells. The secreted protein lacked no amino acid residues at the N terminus with respect to those predicted by the cDNA sequence, except the initiation methionine. The rat FGF-9 cDNA was also cloned, and the structural analysis indicated that the FGF-9 gene is highly conserved.